The invention concerns a method for tuning the probe head impedance of an NMR receiving coil having a receiving circuit for an NMR signal in which the probe head impedance is tuned by means of an RF tuning signal via a first reference signal produced with a real reference resistance at a location M2 within the receiving circuit.
A tuning procedure of this kind is, for example, known in the art through DE 38 38 252 A1.
In the method known in the art for tuning of the NMR receiving coil in the probe head of an NMR spectrometer, the probe head is not tuned by hand separate from the NMR receiver using distinct measuring instruments, rather the NMR receiver is already provided with suitable devices with which this procedure can be automatically carried-out under software control. Thereby the signal-to-noise ratio (S/N-ratio) of the NMR receiver during normal operating conditions should, to the extent possible, not be worsened.
The method known in the art largely utilizes an RF tuning source which delivers the signal for the tuning process, a reference resistance and various switches to automatically switch from the normal operating state into the "probe head tuning" operating state and back again. The reference resistance provides the desired probe head impedance and is switched at that location where the probe head impedance should be tuned to the value of the reference impedance. This reference impedance thereby represents the desired real resistance which should be assumed by the probe head impedance.
The S/N-ratio of the entire receiving system depends strongly on the network in the path of the NMR signal at the input to the preamplifier. Therefore this network should be provided with as few loss-ridden components, for example diodes and chokes, as possible. Since this network is in general low resistance (in the vicinity of 50.OMEGA.) all types of series resistances, for example conducting diodes, should be avoided. Even a series resistance of 1 .OMEGA. can lead to a noticeable deterioration in the S/N-ratio.
Through the utilization of pin diodes as switches RF properties which are normally improved compared to those of other diodes are achieved. Nevertheless a conducting pin diode still has a series resistance of approximately 1 .OMEGA. and thereby causes, in the conducting state, non-negligible RF losses, for example 0.1 db, which is already excessive by today's standards.
In the non-conducting state a pin diode also causes RF losses which, however, are somewhat smaller than in the conducting state. The diode acts, in the non-conducting state, like a small capacitance of approximately 0.5 pF with a parallel loss resistance of approximately 8 k.OMEGA.. A pin diode at the input to the preamplifier should therefore, in the normal operating state of the receiver system, preferably be non-conducting rather than conducting.
The RF choke known and used in prior art is also not an ideal component, since it has an RF impedance similar to that of a non-conducting pin diode, namely a capacitance of approximately 0.5 pF and a parallel resistance of approximately 5 k.OMEGA..
It is therefore the underlying purpose of the present invention to further improve a method of the above-mentioned kind in such a fashion that, in the path of the NMR signal at the input to the preamplifier as few diodes, RF chokes and other components which could lead to signal losses are utilized as possible. Furthermore, in the path of the NMR signal at the input of the preamplifier all diodes which are there present should be non-conducting in the normal operating state and no diodes in series with the path of the NMR signal should be utilized.